1/4" x 1/4" Stainless Steel Twin Ferrule Bulkhead Female Connector Compression x BSP Parallel fittings have been designed for high pressures, vacuums, and where vibration is inherent. These fittings are precisely machined to close tolerances. Manufactured in the UK by a BS EN ISO 9001:2008 registered company. The imperial compression sizes given refer to the size (outside diameter) of the tube which accepts the fitting.